A prior art power storage device is disclosed in Japanese Patent Laid-open No. Hei 8-78060(1996). In this power storage device, two storage battery units are connected in series, and a voltage determining device and a plurality of series-connected resistors are connected across the opposite ends of each of the storage battery units. The voltage determining device provides a voltage indicating whether or not the voltage of the storage battery unit is as high as a predetermined voltage. The series-connected resistors divide the voltage of the storage battery unit to provide a reference voltage.
A comparator compares the output voltage of the voltage determining device with the reference voltage determined by resistance type voltage division. If one of the storage battery units is fully charged and the output voltage of the voltage determining device is higher than the reference voltage, the output of the comparator goes LOW. Then, a FET connected in series to the storage battery unit is turned off to stop charging the storage battery unit.
In this prior art power storage device, the potential levels of the output of the voltage determining devices with respect to the potential level of the lowest negative terminal of the series-connected storage battery units are different. Therefore, the series-connected resistors for determining the reference voltage must be specially formed for the storage battery units. Thus, circuits of the same function conforming to the potential levels of the storage battery units are necessary for the storage battery units.
Since the resistances of the resistors are dispersed in a range around a nominal resistance, the reference voltages indicating a fully charged state differ from each other and the accuracy of the reference voltages is unsatisfactory. Therefore, resistors having resistances close to a desired resistance must be selectively used. Such resistors, however, are expensive.
The withstand voltage of the comparator must be equal to the sum of the voltages of the series-connected storage battery units. Therefore, when many storage battery units are connected in series, an increased number of circuits respectively conforming to the potential levels of those storage battery units are necessary, which increases the cost, size and power consumption of the power storage device and the power storage device needs parts including a comparator having a high withstand voltage.